A work-piece rotating about an axis can be ground into parts which are circularly asymmetric with the axis of rotation by means of a grinding wheel which is moved laterally to that axis. In order to grind the parts rapidly, and to a high degree of geometric accuracy, a high performance servo system is used to control the movement of the grinding wheel towards and away from the work-piece as the latter rotates.
The servo system includes sensors for measuring the position of the wheel and the velocity with which it moves towards or away from the work-piece, and these are connected in respective position and velocity feedback loops which facilitate the accurate control of those parameters by a control computer. Typically, the position feedback loop has a bandwidth of 10 Hz and the bandwidth of the velocity feedback loop is 200 Hz.
The controlling computer can calculate very accurately the exact grinding wheel position required for any angular portion of the work-piece, (the work-piece angle) so as to generate the required offset diameter. The same computer can measure the instantaneous position of the grinding wheel (through the position feedback loop) at any work-piece angle by using a high speed, high accuracy measuring system such as a linear scale to a typical position resolution of 0.0001 mm.
The computer, knowing the required position and the actual position of the grinding wheel can calculate a position error and feed an error signal derived therefrom to the servo system controlling the movement of the grinding wheel, in order to correct its position and thereby minimise the position error for all work-piece angular positions.
In order to achieve the required geometric tolerances in the finished component, grinding wheel position errors of typically less than 0.001 mm are required.
However, it is difficult to maintain the required error envelope to achieve such accuracy, especially if the work-piece is to rotate at speeds of more than 20 rpm, as a result of the limited bandwidth of the position control loop. Furthermore, there is no correction of time varying contributions to the error (for example electronic or servo drift) which can become significant over a protracted period of time.
It is an object of the present invention to provide an improved control system and method for controlling a machine tool so as to achieve the desired error envelope with higher speeds of rotation of the work-piece.